This invention relates to a magnetic recording substance and more particularly, it is concerned with a magnetic recording substance having an excellent dispersibility, surface property and magnetic property, which is obtained by subjecting an alloy fine powder of liquid phase reduction type to a special treatment.
In a magnetic recording substance provided with a magnetic layer on a support, a magnetic paint comprises, as main components, a ferromagnetic powder, binder and coating solvent and, optionally, other additives such as dispersing agents, lubricants and abrasives. Methods of preparing such a magnetic paint are disclosed in Japanese Patent Publication Nos. 186/1968, 28043/1972, 28045/1972, 28046/1972, 28048/1972 and 31445/1972.
Ferromagnetic materials used in magnetic recording substances such as audio tapes, video tapes, memory tapes, magnetic sheets and magnetic cards are fine powders of ferromagnetic iron oxides, cobalt ferrite, ferromagnetic chromium dioxide and ferromagnetic metals or thin films of ferromagnetic metals. These magnetic recording substances have been used in a wide variety of technical field wherein electric or magnetic signals are recorded and reproduced and, of late, a system of recording, in particular, a short wavelength signal in a high density, has been watched with keen interest. Accordingly, magnetic recording properties suitable for high density recording, for example, a considerably high coercive force and large residual magnetic flux density are required for ferromagnetic materials. Moreover, it is necessary, for example, for magnetic cards, that demagnetization due to heating or pressing is be little. Metallic ferromagnetic materials have been considered most promising for high density recording with low demagnetization.
On the other hand, the recording wavelength in a video tape is much shorter than in a sound recording. For example, in a VTR (video tape recorder) for broadcasting, a short wave to a minimum wavelength of 6 microns has to be recorded. However, the above described oxide-type magnetic substances are not as suitable for magnetic recording of a signal of short recording wavelength (about 10 microns or less), because their magnetic properties such as coercive force (Hc) and residual magnetic flux density are insufficient for high density recording. Furthermore, recording and reproducing mechanisms have lately been miniaturized and simplified and, in addition to VTR of rotary head type used in the prior art, a small VTR of fixed head type has been developed. In such a fixed head VTR, it is particularly important to lower the relative speed between a tape and head. That is to say, the minimum recording wavelength is preferbly at most 2 microns or less in order to prevent a video tape from high speed transporting in a small VTR.
Development of ferromagentic metal powders having properties suitable for high density recording has lately been carried out actively. The following methods are known as a method of preparing the ferromagnetic metal powder:
1. A method comprising heat-decomposing an organic acid salt of a metal capable of forming a ferromagnetic material, for example, oxalate and reducing with a reducing gas, for example, hydrogen gas. This method is described in, for example, Japanese Patent Publication Nos. 11412/1961, 22230/1961, 14809/1963, 3807/1964, 8026/1965, 8027/1965, 15167/1965, 16899/1965 (U.S. Pat. No. 3,186,829), 12096/1966, 14818/1966 (U.S. Pat. No. 3,190,748), 24032/1967, 3221/1968, 22394/1968, 29268/1968, 4471/1969, 27942/1969, 38755/1971, 38417/1971, 41158/1972 and 29280/1973.
2. A method comprising reducing a needle-like iron oxyhydride, substance containing metals other than iron as well as the above oxhydride or needle-like iron oxide derived from the oxyhydride. This method is described in, for example, Japanese Patent Publication Nos. 3862/1960 11520/1962, 20335/1964, 20939/1964, 24833/1971, 29706/1972, 30477/1972 (U.S. Pat. No. 3,598,568), 39477/1972, Japanese Patent Application (OPI) Nos. 5057/1971, and 7153/1971, Japanese Patent Publication No. 24952/1973, Japanese Patent Application (OPI) Nos. 79153/1973 and 82395/1973 and U.S. Pat. Nos. 3,607,220, 3,681,018, 3,598,568 and 3.702,270, British Patent No. 1,192,167 and DOS No. 2,130,921.
3. A method comprising evaporating a ferromagnetic metal in a low pressure inert gas. This method is described in, for example, Japanese Patent Publication Nos. 2560/1971, 4131/1972, 27718/1972, Japanese Patent Application (OPI) Nos. 25662/1973 to 25665/1973, 31166/1973, 55400/1973 and 81092/1973.
4. A method comprising heat-decomposing a metal carbonyl compound. This method is described in Japanese Patent Publiction Nos. 1004/1964, 3415/1965 and 16868/1970 and U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007 and 3,228,882.
5. A method comprising electrodepositing a ferromagnetic metal powder using a mercury cathode and then separating the metal powder from mercury. This method is described in Japanese Patent Publication Nos. 12910/1960, 3860/1961, 5513/1961, 787/1964, 15525/1964, 8123/1965, 9605/1965 (U.S. Pat. No. 3,198,717), 19661/1970 (U.S. Pat. No. 3,156,650) and U.S. Pat. No. 3,262,812.
6. A method comprising reducing a solution containing a metal salt capable of forming a ferromagnetic metal powder by adding thereto a reducing agent, for example, borohydride compound, phosphinate or hydrazine. This method is described in, for example, Japanese Patent Publication Nos. 20520/1963, 26555/1963, 20116/1968, 9869/1970, 14934/1970, 7820/1972, 16052/1972, 41718/1972 and 4719/1972 (U.S. Pat. No. 3,607,218), Japanese Patent Application (OPI) Nos. 1353/1972 (U.S. Pat. No. 2,756,866), 1363/1972, 42252/1972, 42253/1972, 44194/1973, 79754/1973 and 82396/1973 and U.S. Pat. Nos. 3,206,338, 3,494,760, 3,535,104, 3,567,525, 3,661,556, 3,663,318, 3,672,867 and 3,726,664.
The present invention is suitable for ferromagnetic metal powders obtained by the methods (4), (5) and (6) of the above described six methods, in particular, obtained by the liquid phase reduction method (6) wherein a borohydride compound or phosphinate is used as a reducing agent.
In the magnetic recording substance of the foregoing type, a ferromagnetic metal powder is dispersed in an organic binder and, in the case of obtaining the ferromagnetic metal powder by the liquid phase reduction method, threfore, it is necessary to remove water therefrom.
As a method of dehydrating such a ferromagnetic metal powder, there are the following methods:
1. A method comprising heating and drying in the air or, if necessary, in an atmosphere of nitrogen.
2. A method comprising drying in vacuo.
3. A method comprising extracting water with an organic solvent miscible with water, for example, alcohol or acetone and then substituting by another organic solvent used in dispersing in a binder. These methods have disadvantages as follows: In the first method, a ferromagnetic metal powder tends to be oxidized or fired during drying and further to absorb moisture during storage after the drying. In the second method, the drying efficiency is low and a ferromagnetic metal powder tends to fire after drying and to absorb moisture during storage. The third method needs large amounts of organic solvents during water extracting and substituting and is thus disadvantageous in respect of the processing cost and effective utilization of resources.